DAHLGREN, THE MALARIA MOSQUITO 33 



part directly on the hard chitinous pieces which compose the upper and 

 lower shields of the abdomen. 



Respiration is carried on by means of a system of air-tubes, or tracheae, 

 which open to the exterior by two main openings on either side of the 

 thorax, and by eight smaller ones in the soft membrane of the abdomen 

 (Fig. 21). The trachea?, by repeated branching, ramify 

 throughout the entire body of the mosquito and supply the 

 blood, as well as every organ and tissue, with air. Nearly filling all 

 spaces between the muscles and the organs, are symmetrically arranged 

 masses of a peculiar tissue, the blood-tissue or fat-body, which is espe- 

 cially well supplied with trachea?. 



The circulation of the blood of the insect is maintained by the heart, 

 which is a tubular organ lying directly under the upper chitin-shields of 

 the abdomen (Fig. 22). It is continued forward in the thorax as a ves- 

 sel, the "aorta," through which the blood is pumped to the 



Circulation 



head. This is the only blood vessel to be found in insects, 

 and the blood circulates with the respiratory movements throughout 

 the body in the interstices between the fat-tissue and the internal organs. 

 Into the blood in the body cavity of the mosquito, the malarial spores 

 which grow in its stomach-wall escape. Through the circulation of the 

 blood the spores then find their way into the salivary or poison glands. 

 These important little glands, which supply the irritating poison of 

 the mosquito bite, lie within the anterior part of the thorax just beyond 

 the neck (Fig. 26). The secretion from each three-lobed gland is con- 

 ducted forward into the head by a fine tube, the salivary duct. 

 t i i i i i • • i i i • Poison 



In the head the two ducts join and the common duct empties Glands 



into the salivary pump (Fig. IS). This, in connection with its 

 continuation, the salivary channel in the hypopharynx, forms a practical 

 syringe by which the poisonous saliva is automatically forced out at the 

 point of the proboscis during the act of feeding. It has been thought 

 that the saliva serves to prevent the clotting of the blood in the mos- 

 quito's sucking tube, but this, strangely enough, seems doubtful. 1 Its 

 irritating effect is however well known, and it is, furthermore, with this 

 salivary secretion that the malarial spores are injected into the human 

 circulation. 



'G. F. H. Nuttall and A. E. Shipley. Jour, of Hygiene, London, 1900, p. 195. 



